Methods known from the past for measuring moisture (water vapor) concentration in a gas include the crystal oscillation method and the electrostatic capacitance method. With the crystal oscillation method, a sensitive membrane that absorbs moisture that is present in a gas is adhered to a crystal oscillator and the change in frequency of the crystal oscillator is measured. With the electrostatic capacitance method, the change in electrostatic capacitance of a sensitive membrane is measured. However, these methods are not suited for the measurement of minute quantities of moisture and also suffer from poor stability of the measurements caused by a drop in measurement accuracy due to factors such as degradation of the sensitive membrane. A moisture measurement device that has been proposed in recent years for measuring moisture concentration in gas uses infrared absorption spectroscopy that is based on the absorption of laser light in the infrared region (see for example Patent Literature 1 and 2).
With this moisture measurement device, laser light having a predetermined wavelength is irradiated onto a sample cell in which gas to be measured is introduced and transmitted laser light is analyzed to determine moisture concentration from the amount of absorption by the moisture in the gas. Because the device is a non-contact type and the light reception unit that serves as the sensor does not contact the gas to be measured, the device, unlike previous devices that employ the crystal oscillation method or the electrostatic capacitance method, can be used for the measurement of moisture in corrosive gases. Furthermore, because moisture measurement can be performed quickly, the device is suited for purposes such as the continuous monitoring of moisture concentration in a flowing gas.
Among such infrared absorption spectroscopy that uses laser light, harmonic detection spectroscopy that uses secondary harmonics is particularly well known (see for example Non-Patent Literature 1). Non-Patent Literature 2 discloses a moisture measurement method that uses harmonic detection spectroscopy. The theory behind moisture measurement according to this literature is briefly explained next.
If vaporized moisture is present in air or nitrogen (gas to be measured) with a gas pressure of 1 atmosphere or more, the shape of the absorption property is represented by a Lorentz profile identified by equation (1) below.
                    Equation        ⁢                                  ⁢        1                                                                                                I              0                        ⁡                          (              v              )                                -                      I            ⁡                          (              v              )                                      =                              1            π                    ⁢                                    P              ⁢                                                          ⁢              c              ⁢                                                          ⁢              L              ⁢                                                          ⁢              S              ⁢                                                          ⁢              γ                                                                        (                                      v                    -                                          v                      0                                                        )                                2                            +                              γ                2                                                                        (        1        )            Here, I0(ν) represents the intensity of the incident light at frequency ν, and I(ν) represents the intensity of the transmitted light at frequency ν. P represents the gas pressure, c the volume concentration of water molecules, L the length of the optical path passing through the gas to be measured, and S the predetermined linear strength of absorption property, γ the half-width of the absorption property, and ν0 the center frequency for the frequency modulation. Equation (2) below represents the absorption intensity I(ν0) at the center frequency ν0.
                    Equation        ⁢                                  ⁢        2                                                                                                I              0                        ⁡                          (                              v                0                            )                                -                      I            ⁡                          (                              v                0                            )                                      =                              1            π                    ⁢                                                    P                ⁢                                                                  ⁢                c                ⁢                                                                  ⁢                L                ⁢                                                                  ⁢                S                            ⁢                                                                    γ                                              (        2        )            
Infrared absorption by water molecules in very low total pressure regions (high vacuum regions where the total pressure of the gas to be measured is no more than 1 Torr) results in the width of the absorption property to be narrower than the width of the aforesaid Lorentz profile by a factor of several-fold to several dozen-fold. The width of the absorption property in said total pressure region is primarily determined by the Doppler effect. The width of the absorption property is represented by a Gaussian line shape expressed by equation (3) below.
                    Equation        ⁢                                  ⁢        3                                                                                                I              0                        ⁡                          (              v              )                                -                      I            ⁡                          (              v              )                                      =                              1                                          γ                ED                            ⁢                              π                                              ⁢                                    P              ⁢                                                          ⁢              c              ⁢                                                          ⁢              L              ⁢                                                          ⁢              S                                                      exp                ⁡                                  (                                                            v                      -                                              v                        0                                                                                    γ                      ED                                                        )                                            2                                                          (        3        )            
In equation (3), γED is referred to as the Doppler width and depends on the center frequency of the absorption frequency, molecular weight and temperature. Here, the absorption intensity I(ν0) at center frequency ν0 is represented by equation (4) below.
                    Equation        ⁢                                  ⁢        4                                                                                                I              0                        ⁡                          (                              v                0                            )                                -                      I            ⁡                          (                              v                0                            )                                      =                                            P              ⁢                                                          ⁢              c              ⁢                                                          ⁢              L              ⁢                                                          ⁢              S                        ⁢                                                                                    γ              ED                        ⁢                          π                                                          (        4        )            Under conditions of a high vacuum and at room temperature of approximately 25° C., with an absorption spectrum in a region of relatively strong absorption that allows the use of an ordinary near-infrared semiconductor laser, γED is approximately equal to 0.01 cm−1. With water molecules that are present in air or nitrogen matrix at 1 atmospheric pressure, the general value of γ is 0.1 cm−1.
Performing harmonic detection requires modulation of the frequency of light that is irradiated onto the gas to be measured. Letting a represent the modulation amplitude of a sine wave signal for frequency modulation and w represent frequency, the frequency of light at time t is defined by equation (5) below.Equation 5ν mod(t)=ν+a·cos ωt  (5)
With a second harmonic detection, signal components that correspond to twice the frequency or 2ω are extracted. The second harmonic detection signal at center frequency ν0 for water molecules that are present in air or nitrogen at 1 atmospheric pressure is defined by equation (6) below.
                    Equation        ⁢                                  ⁢        6                                                                      signal          ⁡                      (                          v              0                        )                          ⁢                  2          π                ⁢                              P            ⁢                                                  ⁢            c            ⁢                                                  ⁢            L            ⁢                                                  ⁢            S                    γ                ⁢                              ∫            0            π                    ⁢                                                    cos                ⁡                                  (                                      2                    ⁢                    θ                                    )                                                                                                  (                                                                  a                        ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                        θ                                            γ                                        )                                    2                                +                1                                      ⁢            d            ⁢                                                  ⁢            θ                                              (        6        )            
Similarly, the second harmonic detection signal at center frequency ν0 for water molecules in a vacuum atmosphere is defined by equation (7) below.
                    Equation        ⁢                                  ⁢        7                                                                      signal          ⁡                      (                          v              0                        )                          ⁢                  2                      π                          ⁢                              P            ⁢                                                  ⁢            c            ⁢                                                  ⁢            L            ⁢                                                  ⁢            S                                γ            ED                          ⁢                              ∫            0            π                    ⁢                                                    cos                ⁡                                  (                                      2                    ⁢                    θ                                    )                                                                              exp                  ⁡                                      (                                                                  a                        ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                        θ                                                                    γ                        ED                                                              )                                                  2                                      ⁢            d            ⁢                                                  ⁢            θ                                              (        7        )            Non-Patent Literature 2 proves that signal (ν0) with the highest sensitivity is obtained when the modulation amplitude a is selected so that a/γ (or a/γED)=2.2 in equations (6) and (7) above.
Even though the afore-described moisture measurement method that uses laser light has advantages over previous measurement methods, it also has the following problems. To explain, laser light passes through not just the gas to be measured but also space occupied by other than the gas to be measured. This results in the moisture (hereinafter “interfering moisture”) derived from the atmosphere that is present in that space to act as background noise that affects the measurement results. To eliminate these effects, what is typically done is to supply a purge gas into the chamber that houses optical system elements such as laser light source and photodetectors to reduce the amount of interfering moisture.
However, because the interfering moisture exists in atmosphere in large quantities, even if the afore-described method is used, to guarantee that the interfering moisture is eliminated with certainty requires that the status of interfering moisture be continuously monitored. This is particularly so with equipment that is used in semiconductor manufacturing processes for monitoring and measuring moisture concentration in a gas for extended periods. With such equipment, it is important to maintain the dehumidifying capacity of purge gas that is supplied in large quantities at a constant level, and this requires measuring the amount of interfering moisture in the purge gas. However, methods or devices for easily measuring interfering moisture have not been proposed previously.